The present invention relates generally to ultrasonic transducers, and, more particularly, to an AC biased micro-machined ultrasonic transducer (MUT) having improved sensitivity.
Ultrasonic transducers have been available for quite some time and are particularly useful for non-invasive medical diagnostic imaging. Ultrasonic transducers are typically formed of either piezoelectric elements or of micro-machined ultrasonic transducer (MUT) elements. The piezoelectric elements typically are made of a piezoelectric ceramic such as lead-zirconate-titanate (PZT), with a plurality of elements being arranged to form a transducer. A MUT is formed using known semiconductor manufacturing techniques resulting in a capacitive ultrasonic transducer cell that comprises, in essence, a flexible membrane supported around its edges over a silicon substrate by an insulating material. By applying contact material, in the form of electrodes, to the membrane, or a portion of the membrane, and to the base of the cavity in the silicon substrate, and then applying appropriate voltage signals to the electrodes, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, when electrically biased, the membrane of the MUT may be used to receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the electrically biased membrane, which then generates a receive signal.
The ultrasonic transducer elements may be combined with control circuitry forming a transducer assembly, which is then further assembled into a housing possibly including additional control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include various acoustic matching layers, backing layers, and de-matching layers may then be used to send and receive ultrasonic signals through body tissue.
MUT arrays are typically designed where each MUT element is a transceiver. In such an arrangement, each MUT element both produces a transmit pulse and receives acoustic energy. Unfortunately, the characteristics of a MUT element that make it a good transmitter of acoustic energy are not the same characteristics that make it a good receiver of acoustic energy. For example, during a transmit pulse, it is desirable for the MUT to provide a large power output. To accomplish this, a large membrane deflection, a large gap, high membrane stiffness, and high bias voltage are used to produce the high pressure wave desired on transmit. In such a MUT, the cavity depth should be at least three times deeper than the static deflection of the membrane. Membrane deflection larger than approximately ⅓ of the cavity depth results in the collapse of the membrane against the cavity floor. The gap is defined as the distance between the membrane and the bottom of the cavity. A large gap results in a small capacitance and large imaginary impedance.
Conversely, for a MUT to be a sensitive acoustic receiver, a small membrane deflection, a small gap, low membrane stiffness, and high bias voltage are used to produce a sensitive acoustic receiver element. In the past, a DC bias voltage has typically been applied to deflect the membrane and reduce the gap to the minimum uncollapsed size. The small gap reduces the imaginary impedance and the soft membrane deflects easily when exposed to acoustic energy reflected from a target resulting in a high signal-to-noise ratio (SNR). During receive operation the DC bias voltage functions as a xe2x80x9csensexe2x80x9d voltage and the current (I) through the MUT is monitored so that the capacitance (C) of the MUT can be measured. The charge (Q) on the MUT is defined as Q=C*V, where C is the capacitance of the MUT and V is the DC bias voltage applied to the MUT. The current (I) is defined as I=dQ/dt,=d[Cxc3x97V]/dt.
Unfortunately, the application of a DC bias voltage to the MUT during receive operation has drawbacks. For example, in order to increase the receive sensitivity of the MUT, the DC bias voltage should be increased. Unfortunately, once the DC bias voltage reaches a certain point, commonly referred to as the collapse voltage, Vcollapse, the MUT membrane collapses against the floor of the cavity and becomes inoperable as a receiver. Therefore, when using a DC bias voltage, the sensitivity of the MUT is limited by I=dC/dt*Vcollapse.
Therefore, it would be desirable to have the ability to adjust the sensitivity of a MUT without this limitation on bias voltage.
An AC biasing arrangement for a micro-machined ultrasonic transducer (MUT) is disclosed. The AC biasing arrangement allows the sensitivity of each MUT element in an array to be adjusted without collapsing the MUT membrane. The sensitivity of the MUT element can be adjusted by varying the frequency of the AC bias signal supplied to the MUT element. An alternative embodiment of the invention adds a second AC bias signal having a phase opposite the phase of the first AC bias signal. This arrangement provides a neutral bias signal, thereby removing the large amplitude bias signal from the MUT element.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.